📘 Three Address Code (TAC)

(Compiler Construction — exam-ready, simple explanation with examples, rules, and key points)

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🧠 1. What is Three Address Code (TAC)?

✅ Definition

Three Address Code (TAC) is an intermediate representation (IR) used in compilers where each instruction has at most three addresses (operands).

👉 It breaks complex expressions into simple step-by-step instructions.

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📌 General Form

x = y op z

Where:

• x → result
• y, z → operands
• op → operator (+, -, *, /, etc.)

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🎯 2. Why TAC is Used?

✔ Simplifies complex expressions ✔ Helps in optimization ✔ Easy to convert into machine code ✔ Machine-independent

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🔧 3. Structure of TAC Instructions

TAC supports different types of statements:

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🔹 1. Assignment

x = y

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🔹 2. Binary Operation

x = y + z

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🔹 3. Unary Operation

x = -y

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🔹 4. Conditional Jump

if x < y goto L1

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🔹 5. Unconditional Jump

goto L1

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🧠 4. TAC for Expressions

📌 Example 1:

Expression:

a = b + c * d

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🔄 Step-by-step TAC:

t1 = c * d
t2 = b + t1
a = t2

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🌳 5. TAC using Precedence Rules

👉 Multiplication first, then addition

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📌 Example 2:

x = (a + b) * (c - d)

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TAC:

t1 = a + b
t2 = c - d
t3 = t1 * t2
x = t3

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🔄 6. TAC for Boolean Expressions

📌 Example:

if (a < b && c < d)

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TAC:

if a < b goto L1
goto L2
L1: if c < d goto L3
goto L2
L3: ...

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📌 7. TAC for Control Statements

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🔹 If Statement

if (x > 0)
    y = 1;

TAC:

if x > 0 goto L1
goto L2
L1: y = 1
L2:

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🔹 If-Else Statement

if (x > 0) y = 1;
else y = 2;

TAC:

if x > 0 goto L1
goto L2
L1: y = 1
goto L3
L2: y = 2
L3:

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🔹 While Loop

while (x < 10)
    x = x + 1;

TAC:

L1: if x >= 10 goto L2
    x = x + 1
    goto L1
L2:

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🧠 8. Types of TAC Representation

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🔹 1. Quadruples

Format:

(op, arg1, arg2, result)

Example:

op	arg1	arg2	result
*	c	d	t1
+	b	t1	t2
=	t2		a

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🔹 2. Triples

Format:

(index) op arg1 arg2

Example:

i	op	arg1	arg2
0	*	c	d
1	+	b	(0)
2	=	(1)	a

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🔹 3. Indirect Triples

• Uses pointer table
• Easier optimization
• More flexible than triples

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🔄 9. TAC vs High-Level Code

Feature	High-Level Code	TAC
Complexity	High	Simple
Structure	Complex	Linear
Optimization	Hard	Easy
Machine dependency	No	No

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🎯 10. Advantages of TAC

✔ Simplifies expressions ✔ Easier optimization ✔ Helps code generation ✔ Machine-independent

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❌ 11. Disadvantages

❌ Needs extra memory (temporary variables) ❌ Not executable directly ❌ Requires further translation

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📌 12. Important Exam Points

👉 Frequently asked:

• Define Three Address Code
• Convert expression to TAC
• TAC for if-else / loops
• Quadruples vs triples
• Why TAC is used in compiler
• Advantages of TAC

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📝 13. Short Notes (Quick Revision)

🔵 TAC

• Intermediate code
• At most 3 operands
• Uses temporaries

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🔵 Forms

• Quadruples
• Triples
• Indirect triples

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📊 14. Final Summary Table

Aspect	Three Address Code
Type	Intermediate representation
Format	x = y op z
Purpose	Simplify code
Output	Linear instructions
Use	Optimization & code generation
Machine dependency	No

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✅ Final Conclusion

• Three Address Code (TAC) is the most important intermediate representation in compilers
• It breaks complex expressions into simple instructions using temporary variables
• It is widely used for optimization and machine code generation

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